The manufacture of printed circuit boards ("PCB's") often involves mounting of leaded-through-hole, surface-mount leaded, and leadless electronic components to one or both sides of an insulating substrate. Processing techniques are known to consist of a number of precisely orchestrated steps which can include, adhesive applying, component positioning, and elaborate soldering operations.
Typically, leaded-through-hole components are dropped onto the component side of a printed circuit board and inserted into plated through-holes in the board. The leads of these components are then soldered by exposing the lower surface of the board to a solder wave or dip solder pool which wets both the leads and the conducting surfaces around the leads to secure the leaded-through-hole components to one or more conducting surfaces of the printed circuit board.
In an effort to increase the density of electronic components upon these boards, electronics manufacturers have recently been employing double-sided-mounting techniques in which electronic components are mounted to both sides of a single printed circuit board. With such techniques, both leaded-through-hole and surface-mount components can be soldered to the component side of a printed circuit board with additional surface-mount components being attached to the bottom, or solder side, of the board.
The art has been replete with developments for double-sided-mounting techniques. See Berger, U.S. Pat. No. 4,515,304, (hereinafter "Berger"), Sagawa et al., U.S. Pat. No. 4,814,944, (hereinafter "Sagawa"), Beldavs, U.S. Pat. No. 4,573,105, (hereinafter "Beldavs"), Roback, et al., U.S. Pat. No. 4,851,966, (hereinafter "Roback"), and Bora et al., U.S. Pat. No. 4,761,881, (hereinafter "Bora"), all of which are hereby incorporated by reference as part of this disclosure.
Berger discloses a process for mounting leaded components on one side of a printed circuit board and surface-mount components on the other side of the board using solders having different melting temperatures. This technique employs crimping the lead ends of the leaded-through-hole components prior to flipping over the board and disposing solder paste onto the bottom side by individual applicator nozzles. The paste is applied to the crimped lead ends of the through-hole components and to the surface-mount solder pads on the bottom side of the board. The surface-mount parts are then positioned on the surface-mount pads and the board is reflow soldered. This invention relies on employing a paste solder having a lower melting temperature on the top side of the board for permitting a second reflow soldering operation without losing components on the bottom side. Wave soldering is completely avoided.
Sagawa discloses a lead terminal foot configuration for facilitating the soldering of a surface-mount component to a printed circuit board. This reference teaches the well known technique of provisionally securing the surface-mount component to the printed circuit board with adhesive prior to soldering.
Beldavs discloses a procedure for securing leadless surface-mount components to a printed circuit board which includes bonding the dielectric body of the component to a hole formed in the board with an adhesive plug, hardening the adhesive, and then mass soldering the component to the circuit board.
Roback discloses a method for optimally placing microminiature components on a printed circuit board which includes applying a solder paste to the top side of the board before placing components thereon. These components are then reflow soldered, during which, any misaligned components can be repositioned. Additional components are then attached to the underside of the circuit board. These components are first self-aligned by specially contoured soldering pads, and then affixed in place with adhesive. The underside components and any leaded-through-hole components extending through to the bottom side are then wave soldered to complete the assembly.
Bora discloses a single step reflow soldering process for attaching a variety of component types to both sides of a printed circuit board. The lands of the surface-mount components of the bottom surface are initially screen soldered and then adhesive is dispensed at each component site. The components are placed into the wet solder paste and adhesive, and the adhesive is cured. This curing holds the components to the printed circuit board and permits the solder paste to dry. The board is then flipped to the top side and the surface-mount lands and plated through holes on the top surface are coated with solder paste. The top side components are then placed into the wet solder and the solder is reflowed to complete the assembly.
While such prior art double-sided techniques have generally provided for the mass production of high density component boards, these processes have either been time consuming and expensive, or remiss in eliminating missing bottom side components or solder skips. Moreover, reliance upon large amounts of glue to secure the bottom side components prior to soldering, not only complicates the glue dispensing process, but also increases the likelihood of glue contamination of the solder pads.